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Abstract

The overarching goal ofthis project,Neurocognitive Development,has beento contribute to a deeper understanding of normal brain maturation from childhood to adulthood and how this relates to cognition.Although the structural maturation of the brain is beginning to be bettercharacterized by magnetic resonance imaging (MRI) studies,and cognitive developmenthas received much attention for decades, studies on the relationships between different MRI measures and between suchindices of neuroanatomy and cognitive functions in development are still scarce. In three papers, the present thesis examinesstructural brain maturation and its relationships with behavioral performance indices of so-called higher-order cognitive functions, more specifically general intellectual abilities and different selected executive functions. The thesis demonstrates mostly nonlinear age-related cortical thinning, mainly linear white matter (WM) volume increases and decelerating changes in diffusiontensor imaging (DTI)parameters in the age-range 8-30 years. All three classes of measureswere sensitive to brain maturation and showed unique associations with age. Furthermore, the results indicate that the apparent cortical thinning in adolescence cannotbe explained by WM maturation in subjacent regionsas measured by volumetry or DTI. The thesis further demonstrates age-independent relationships betweencognitive functions andMRI and DTI indices,respectively,and also indicatesassociations between brain maturation and cognitive functions.Independently of age, both verbal and performance intellectual abilities were related to DTI indices of WM microstructure, predominantly in the left hemisphere. Further, verbal, but not performance abilities, were associated with WM microstructure maturational differences in widespread regions. This complements previous findings of the significance of cortical maturation for general intellectual abilities. Furthermore, using a battery of six tasks proposedto index the executive functions working memory updating, inhibition and shifting, the thesis demonstrates negative age-independent relationships betweencortical thickness and performance on two tasks thought to reflectupdating and inhibitionfunction, respectively. Also, individual performance differenceson two tasksassumedto indexinhibition ability were associated with differences in estimated cortical maturation in posterior brain regions, but no such effects were found in the prefrontal cortices. In sum, this thesis contributesto the description and understanding of brain maturation and how this relates to,and likely is important for,the development of selected higher-order cognitive functions.